(1) Field of the Invention
The invention relates to gas turbine engines. More particularly, the invention relates to the mounting of low pressure compressor exit stators to turbine engine intermediate cases.
(2) Description of the Related Art
FIG. 1 shows a gas turbine engine 20 having a case assembly 22 containing concentric high and low pressure rotor shafts 24 and 25. The shafts are mounted within the case for rotation about an axis 500 which is normally coincident with central longitudinal axes of the case and shafts. The high pressure rotor shaft 24 is driven by the blades of a high pressure turbine section 26 to in turn drive the blades of a high pressure compressor 27. The low pressure rotor shaft 25 is driven by the blades of a low pressure turbine section 28 to in turn drive the blades of a low pressure compressor section 29 and a fan 30. Air passes through the engine along a core flowpath 502 sequentially compressed by the low and high compressor sections 29 and 27, then passing through a combustor 32 wherein a portion of the air is combusted along with a fuel, and then passing through the high and low turbine sections 26 and 28 where work is extracted. Additional air is driven by the fan along a bypass flowpath 504.
FIG. 2 shows the core flowpath 502 at the downstream end of the low pressure compressor section. A final ring of compressor blades 50 is mounted to an aft compressor disk 52 of the low speed spool. Upstream of the blades 50 is a ring of vanes 54 secured at their outboard ends to a compressor case assembly 56 and at their inboard ends having a seal system for sealing with the low speed spool. Downstream of the vanes 50 is an exit stator 60 having an array of vanes 62 extending between inner (inboard) and outer (outboard) stator shrouds 64 and 66. The stator shrouds have respective outboard and inboard surfaces 67 and 68 which locally form inboard and outboard boundaries of the core flowpath. At downstream ends, the shrouds 64 and 66 have mounting flanges 70 and 71 bolted to associated flanges 72 and 73 respectively extending inward and outward from respective forward portions of respective inboard and outboard walls 74 and 75 of an intermediate case 76. The inboard and outboard walls 74 and 75 (although not necessarily inboardmost and outboardmost) are connected by an array of webs or struts 77. In the exemplary embodiment, a bearing support 80 is also bolted to the flange 72 outboard of a bearing compartment.
The intermediate case 76 is an important structural element of the engine providing a load path for the engine thrust and providing transverse stiffness. Exemplary intermediate cases are formed essentially as castings with subsequent machining and addition of minor components such as threaded inserts for receiving the bolts. The shrouds 64 and 66 are subject to different loads. Although the shrouds may be of like composition (e.g., titanium alloy) to the intermediate case, they may advantageously be made in different ways (e.g., stamping of sheet stock or forging) to provide the desired strength parameters. In an exemplary method of engine assembly, the stator vanes may be preassembled to the shrouds and the stator then bolted to the intermediate case as a unit. The preassembly may involve inserting the vanes through apertures in the shrouds, with a stablug portion 84 at the tip of the vane airfoil protruding beyond the outboard surface of the outboard shroud and being sealed thereto by an encapsulant such as RTV Silicone™. At the inboard end of the airfoil, a transversely extending foot 86 may have an outboard surface facing the inboard surface of the inboard shroud (e.g., contacting). The foot may be secured to the shroud via fasteners such as rivets (not shown).